The human body has electrical characteristics which can be measured for characterizing organ function and for the application of different therapies. For instance, the heart is a complex network of nerve and muscle tissue which operates in synchrony to pump blood throughout the body. Cardiac function may be monitored by sensing the electrical signals naturally conducted at certain places in the heart.
Sometimes it is convenient to apply signals to the body to determine the function of the organs of the body. For example, in ultrasound measurements a sound wave is transmitted into the body and the resulting reflections of the sound are used to image internal organs or a fetus.
Another way to apply signals is to use an implanted series of electrodes which apply a known current and measure the resulting voltage. The relationship between applied current and measured voltage is known as impedance. Thus, impedance is measured by injecting a known current using electrodes and monitoring the electrical voltage required to pass the known current between electrodes. The higher the magnitude of impedance, the higher the magnitude of voltage measured across the load for a known current magnitude.
If the electrodes are placed such that the impedance is measured across a right ventricular portion of the heart, then the impedance measured is a function of the stroke of the right ventricle. The stroke volume of the right ventricle provides a measure of the blood volume pumped by the heart into the lungs in one stroke.
The change in impedance is due to the conductive nature of blood and its changing volume in the left ventricle between contractions. The measured impedance will vary depending on the placement of the electrodes. For example, as shown in FIG. 1A and FIG. 1B, if a current is conducted between the housing of an implantable device 12 and a tip electrode 13 on the end of a catheter 14 with the tip electrode 13 positioned in the apex of the right ventricle 15, then the impedance observed between two electrodes, 16 and 17, located within the right ventricle (and before the tip electrode 13) will measure an increased impedance for a contracted ventricle (systole--FIG. 1B) as opposed to when the ventricle is not contracted (diastole--FIG. 1A). This is because in diastole, the ventricle is holding more blood and has more conductive volume to transfer current. In systole, the ventricle is contracted and has less blood, leaving less volume for conduction.
Impedance-based measurements of cardiac parameters such as stroke volume are known in the art. U.S. Pat. No. 4,674,518, issued to Salo, discloses an impedance catheter having plural pairs of spaced surface electrodes driven by a corresponding plurality of electrical signals comprising high frequency carrier signals. The carrier signals are modulated by the tidal flow of blood in and out of the ventricle. Raw signals are demodulated, converted to digital, then processed to obtain an extrapolated impedance value. When this value is divided into the product of blood resistivity times the square of the distance between the pairs of spaced electrodes, the result is a measure of blood volume held within the ventricle. These calculations may be made using spaced sensors placed within a catheter, as in the Salo '518 patent, or they may be derived from signals originating in electrodes disposed in the heart, as described in U.S. Pat. No. 4,686,987, issued to Salo and Pederson. The device of the '987 patent senses changes in impedance to determine either ventricular volume or stroke volume (volume of blood expelled from the ventricle during a single beat) to produce a rate control signal that can be injected into the timing circuit of another device, such as a cardiac pacer or drug infusion pump. In this manner, the rate of operation of the slaved device may be controlled. An example of application of this impedance sensing circuitry to a demand-type cardiac pacer is disclosed in U.S. Pat. No. 4,773,401, issued to Citak, et al.
However, many existing measurement systems in biomedical applications provide a continuous excitation of the tissue, and therefore current excitations must be carefully applied to avoid a current which would be unsafe or to avoid quickly depleting the batteries in an implantable device.
Thus, there is a need in the art for a low power signal processing system. The signal processing system should be flexible to provide low power processing of signals in biomedical applications, such as in the measurement of signals related to cardiac performance in implantable devices. In biomedical applications, the signal processing system should operate without requiring unsafe excitation signals and excessive power drain.